The push-pull method is one of the methods for deriving a tracking error signal. Referring to FIGS. 8a to 8c, the push-pull method uses change in distribution of energy in a beam spot which is caused by light diffracted and reflected by a pit P on a disc D when a laser beams is deflected from a track of the disc. When the laser beam is properly centralized on the track, the light is equally diffracted to the right diffracted light and the left diffracted light as shown in FIG. 8b. Thus energy is equally distributed. On the other hand, if the tracking is off-center as shown in FIGS. 8a and 8c, the beam is asymmetrically diffracted. By obtaining the difference between the distributions of energy, it can be determined the direction in which the beam is deflected from the track.
Referring to FIG. 9, a conventional track-following servo system using the push-pull method has a photodetector 4 for detecting a spot S of the reflected beam. The photodetector 4 has two detecting areas 2 and 3 which are defined by a central boundary line 1 in the tangential direction of the disc. The spot S of the reflected beam has a shadow shown by a hatched area in FIG. 9 on each of the detecting areas 2 and 3. The outputs of the detecting areas are applied to a differential amplifier 5 which applies a positive or negative tracking error signal to a track-following servo circuit 6. The circuit 6 operates an actuator 7 for positioning the optical pickup to render the difference of the outputs of the detecting areas 2 and 3 zero.
The outputs of the photodetector 4 are further applied to a summing amplifier 9 to produce an RF signal.
If the track on the disc is properly followed, diffracting the beam as shown in FIG. 8b, the shadows in the spot formed on the detecting areas 2 and 3 have the same area so that the difference between the outputs is zero. If the beam is deflected to the left of the pit, thereby giving a diffraction shown in FIG. 8a, the shadow in the spot on the detecting area 2 is smaller than the shadow on the detecting area 3. To the contrary, if the beam is deflected to the right, so that the beam diffracts as shown in FIG. 8c, the shadow in the spot of area 3 becomes smaller than the shadow on the area 2. Thus a difference is obtained by the differential amplifier 5, thereby applying a tracking error signal to the track-following servo circuit 6. As a result, the actuator 7 is operated to control the error to zero.
In the push-pull method, when the axis of the laser beam is not vertical to the recording surface of the disc, or the objective is moved, or the axis of the laser beam is deflected upon the track-following operation, the spot of the reflected beam moves in the direction perpendicular to the center line 1. Consequently, the distribution of the energy of the spot received by the detecting areas 2 and 3 changes, thereby causing the tracking error signal to have a DC offset. The track-following servo circuit 6 is operated in accordance with the erroneously offset tracking error signal so that the beam is further deflected from the track.
In order to reduce the DC offset, a part of the photodetector 4 is masked to render the size of the detecting areas 2 and 3 smaller than the spot as shown by a reference M in FIG. 9. Elliptic masks M1 and M2 may be provided as shown in FIG. 10.
However, since each mask must be designed to conform with the reflected beam, which is very small, it is difficult to provide an appropriate mask. Furthermore, the spot size of the light beam must be adjusted in accordance with the mask, which is also quite difficult. In addition, the masking method can not perfectly prevent the DC offset.